The Arabian Shield Lithostratigraphic and Lithodemic
Code
Dr. Ahmad Redaa
2026-02-02
Learning outcomes
By the end of this lecture/handout, you should be able to:
- Distinguish lithostratigraphic vs
lithodemic units (and when to use each).
- Explain why the Arabian Shield requires a tailored code for mapping
and nomenclature.
- Apply a decision workflow to classify units in arc terranes and
deformed belts.
- Recognize how terranes and shear/fault
systems constrain unit names and correlations.
- Use correct ranks, naming rules,
and what-not-to-do pitfalls.
What is the ASLLC?
The Arabian Shield Lithostratigraphic and Lithodemic Code
(ASLLC) is a set of rules and procedures for classifying,
ranking, and naming geological units of the Precambrian Arabian
Shield.
It provides: - standardized definitions of rock units, - clear
hierarchical ranks (formation, lithodeme, suite, etc.), - consistent
naming conventions, - a framework compatible with modern digital
geological mapping.
Why was it established?
The ASLLC was developed to support the nationwide remapping
of the Arabian Shield at 1:100,000 scale, because:
- Older stratigraphic schemes were largely informal and
inconsistent
- Many units mixed different rock types, ages, and
origins
- Classical stratigraphy is inadequate for arc-dominated,
deformed Precambrian crust
- Intense deformation obscures original stratigraphic
relationships
- Modern mapping requires database-ready, reproducible unit
definitions
Why a stratigraphic code?
A stratigraphic code is a rulebook for:
- defining rock units,
- assigning rank and category,
- naming units consistently,
- ensuring reproducible mapping and communication.
In modern mapping programs, the code also ensures database
interoperability: unit codes link maps to geochemistry,
geochronology, structural data, and field observations.
Why ASLLC? The Arabian Shield mapping problem
The Arabian Shield is dominated by Neoproterozoic arc-related
rocks formed during accretion and amalgamation.
Common challenges in the Shield:
- intimate relations between plutonic, subvolcanic, and
volcanic facies,
- many lithologically similar arc packages in a condensed time
window,
- limited crosscutting relations at map scale,
- localized but intense deformation (e.g., major shear zones),
- the need to remap consistently at 1:50,000 field
scale, published at 1:100,000.
The big idea: two categories of units
ASLLC deals with formal subdivision of two
categories of rock units:
- Lithostratigraphic units (generally
layered/stratiform)
- Lithodemic units (generally
non-layered/non-stratiform)
A major additional development is a structural
scheme for highly deformed rocks, and a mixed
scheme using “complex” when mapping components separately is
impractical.
Decision question (the most important in ASLLC)
Which scheme do I use? Ask:
- Are primary depositional/volcanic structures preserved?
- Can you establish way-up / stratigraphic order?
- Does the body generally obey Law of
Superposition?
- Is deformation pervasive enough to transpose original features?
- Can components be mapped separately at the working scale?
Outcome:
- Yes → Lithostratigraphic
- No → Lithodemic
- Mixed & unmappable → Complex
- Transposed by high strain → Structural units
(structural lithodemes + zones/systems)
Lithostratigraphy: what it is
Lithostratigraphy classifies
stratiform volcanic and sedimentary rocks based on:
- lithological character,
- relative stratigraphic position.
Typical properties:
- layered/tabular,
- primary structures preserved,
- superposition generally applies.
Low-grade metamorphic equivalents can still be
lithostratigraphic if primary structures and way-up criteria are
preserved.
Lithostratigraphic hierarchy (ranks)
From highest to lowest:
- Supergroup
- Group
- Subgroup
- Formation (fundamental unit)
- Member
- Bed (sedimentary) / Flow
(volcanic)
Formation is the key mapping unit: identifiable, and
mappable.
Arabian Shield context: “supracrustal rafts” and correlation
In arc terranes, supracrustal sequences commonly occur as
isolated rafts intruded by related plutons.
ASLLC guidance:
- Avoid proliferation of names: correlate rafts into a single
formation where defensible.
- Correlation evidence:
- superposition is obeyed,
- significant similar lithic components,
- similar stratigraphic order.
Lithodemics: what it is
Lithodemics classifies rock bodies that are:
- intrusive,
- highly metamorphosed/metasomatized,
- pervasively deformed,
- or otherwise non-stratiform.
Key contrast:
- Lithodemic units generally do not obey
superposition.
- They are defined by observable lithic features and
mapping practicality.
Lithodemic hierarchy (ranks)
From highest to lowest:
- Supersuite
- Suite
- Subsuite
- Lithodeme (fundamental unit)
Notes:
- Units below lithodeme rank are informal.
- Suites express natural relationships among lithodemes (broadly
cogenetic).
Lithodeme: definition and naming
A lithodeme is the fundamental lithodemic unit:
- intrusive / high-grade metamorphic / pervasively deformed rock
body,
- typically non-tabular,
- primary depositional structures absent,
- mappable at the surface.
Naming pattern: Geographic term + lithic
term
Examples: Malik Granite, Abt Schist,
Dumaym Diorite.
Do not add “Lithodeme” to the name (e.g., not “Abt
Lithodeme”).
Suite and Supersuite: what they mean
A Suite groups ≥2 associated lithodemes of the same
class (intrusive or metamorphic).
- Comparable (cartographically) to a lithostratigraphic
Group.
- A Subsuite may group a subset of lithodemes in a
suite if useful.
A Supersuite groups ≥2 suites/complexes with broader
natural relationship.
Important modernization:
- “Igneous complex” (old usage) is obsolete for
grouping associated intrusives.
- Use Intrusive Suite / Metamorphic
Suite.
Complex: when you are allowed to use it
A Complex is used only when mapping each component
separately is impractical at the working scale.
Three accepted complex types:
- Complex (generic): mixed classes and/or mixed ages
where components cannot be separated.
- Volcanic Complex: diverse, cogenetic extrusive
volcanic + volcano-sedimentary assemblage (often rank-like a
subgroup).
- Intrusive Complex: variable, coeval intrusive
assemblage that cannot be separated (often rank-like a subsuite).
Key caution: - Do not use “complex” simply because rocks look
complicated. - It is a cartographic and subdivision tool, not a
shortcut.
Definitions: shear zone vs fault vs fault zone
- Shear Zone: planar zone of localized strong
deformation; includes ductile zones and brittle faults.
- Fault: discrete brittle discontinuity with
displacement, mapped as linear feature.
- Fault Zone: polygonal zone of closely spaced
faults/dominantly brittle deformation.
Systems (rank above): - Shear Zone System / Fault
System must include ≥2 named zones/faults and reflect a broadly
coeval tectonic regime.
Terranes and tectonostratigraphic position: why unit names may be
terrane-limited
The Arabian Shield is a mosaic of originally allochthonous
terranes.
Implication for naming/correlation:
- Units formed before terrane amalgamation are
typically restricted to their terrane/boundary zones.
- Units formed after amalgamation that span multiple
terranes should share stratigraphic designations across those
terranes.
This protects against false correlation of unrelated arc
packages.
Naming rules you must follow
Formal names are compound.
Lithostratigraphic units
- Geographic + rank:
Qarfa Formation
- Geographic + lithic + rank (optional):
Qarfa Rhyolite Formation
- Groups do not include lithic term:
Bayda Group
Lithodemic units
- Lithodeme:
Malik Granite (no “formation”, no
“lithodeme” word)
- Suite:
Abanat Intrusive Suite /
Someplace Metamorphic Suite
- Complex:
Someplace Complex,
Someplace Intrusive Complex,
Someplace Volcanic Complex
Capitalization: - First letter of each word in formal unit names is
capitalized.
What ASLLC explicitly does NOT do (avoid these errors)
ASLLC is not:
- chronostratigraphy (time units are not the basis of
definition),
- magnetostratigraphy / biostratigraphy / allostratigraphy, etc.,
- a tectonic classification system.
Terms such as arc, ophiolite,
belt, pluton, ring
complex are generally informal descriptors.
They may appear in legends or free-text notes but are not formal
database unit terms.
Why this matters for digital mapping and exploration
ASLLC is designed for a modern digital mapping workflow:
- unique unit codes enable relational linking between:
- geology polygons,
- geochemistry,
- geochronology,
- structures,
- field stations and samples.
Practical value: - clearer regional synthesis, - improved
reproducibility, - stronger framework for mineral and energy exploration
targeting.
Worked decision example (conceptual)
Scenario: An arc volcanic sequence occurs as rafts,
intruded by tonalite–granodiorite, cut by a major shear zone.
Classification:
- Stratiform volcanics + sediments with preserved bedding →
formations (lithostratigraphy).
- Tonalite–granodiorite pluton with gradational internal contacts →
lithodeme with compound lithic term if needed.
- A mixed contact zone that cannot be separated at mapping scale:
- if multiple coeval intrusions → intrusive
complex
- if mixed classes (intrusive + supracrustal) →
complex
- Where transposed in the shear zone:
- identifiable protolith → same geographic name + structural
descriptor + new code
- unidentifiable protolith → structural lithodeme,
within a shear zone (and potentially a shear zone
system)
Arabian Shield Lithostratigraphic and Lithodemic Code (ASLLC)
Image generated by NotebookLM
Saudi Geological Survey Special Publication Report SGS-SP-2024-2
(ASLLC, 2024).